The lost wax process is said to date back to antiquity. Essentially, the process involves fabricating an expendable model of the object sought to be cast from metal. This pattern, made from material such as wax which is typically meltable or dissolvable, is surrounded with ceramic mold material (the "investment") which has the property of being able to retain its shape. After the ceramic material is formed, the wax pattern is melted out, thereby leaving within the ceramic material a cavity having the contour of the part sought. Of course, proper compensation is made for solidification shrinkage of the pattern material and molten metal.
A variation of the lost wax process has been found to be especially useful in the construction of ultra-lightweight radomes. In this application, the part sought to be made is hollow and the walls of the article are formed around and supported by the wax mold. After the material used to form the hollow article has hardened, the article is heated and the wax melts with little or no wax coating the inner wall of the cast article.
To provide a lightweight radome with good transmission characteristics, sandwich structures are commonly used. The core of the sandwich structure is usually of honeycomb, foam, or fluted core. The facings of the sandwich are normally composed of a binder of polyester, epoxy or polyimide resin reinforced with a glass or fused silica cloth. New methods for fabricating radomes that will operate at temperatures of 500.degree. F. to 700.degree. F. and possibly up to 1000.degree. F. are required. These temperature requirements are created by aerodynamic heating of radomes due to high speed of aircraft and by RF power absorption heating by high power electronic countermeasures antennas. Radomes constructed with honeycombs and adhesive films are presently limited for long-term exposure to temperatures less than 500.degree. F. Honeycomb core radomes, even for low temperature applications, have had a history of moisture entering the cells through pin holes or damaged areas in the facings. The moisture can then migrate through large sections, causing structural and antenna pattern degradation. Physical damage to the radome occurs if the water freezes in the cells and expands.
The problems with the honeycomb core become more acute with high temperature radomes because the high temperature resins are more porous and more difficult to seal. In this later case, the moisture penetrating the core can be converted to ice at low temperatures or steam if the radome temperature exceeds the boiling point of water.
The type of process having the greatest potential for high temperature sandwich radomes is the fluted core process. This process provides the channels necessary for moisture drainage, air expansion and convection cooling of the radome wall. This process also allows the same resin used in the facings to be used in the core and allows the parts to be fabricated in a single stage.
U.S Pat. No. 2,755,216 issued to Lemons describes a process for forming a multi-ducted shell for use as a radome. The multi-ducted shell has an ultra-lightweight construction and is made out of fiberglass or other glass cloth sheet material impregnated with a thermosetting resin. The impregnated sheet is easily formable while wet and the thermosetting resin cures in situ around the wax mold into a final rigid shape at a temperature below the softening or melting temperature of the wax (125.degree.-225.degree. F.).
U S. Pat. No. 4,155,970 issued to Cassell describes a method for making a hollow composite using a destructible core. Cassell recognizes that the process described in U.S. Pat. No. 2,755,216 issued to Lemons, discussed supra, is unsuitable for high temperature radomes because candidate waxes melt at about 125.degree. F. and the high temperature resins harden at about 325.degree.-350.degree. F. Cassell, therefore, uses a heat shrinkable material, such as polytetrafluoroethylene (PTFE), to surround a lead core. This allows lead to be used instead of wax. Lead has been found to be a good material for lost wax casting because it (a) holds its cross-sectional area, (b) is sufficiently malleable to form the radome contours, and (c) has a melting temperature within the proper range for proposed resin curing. The PTFE coating was required because when an uncoated lead mandrel was used as a mandrel to construct the hollow core, small particles of lead were left in the radome following its melting and removal. These lead particles were found to have a deleterious effect on the electronic transmission and reception of the radome.
The primary problem with PTFE mandrels is in their removal after molding the fluted sandwich construction. A very slight depression or dent in the panel may cause the removal force to be increased above the strength of the composite and rupture can occur. In this case, the column wall thickness would have to be increased above service load requirements to accommodate mandrel removal.
U.S. Pat. No. 4,712,605 issued to Sasaki et al describes a process for producing hollow cast articles. The process comprises the steps of preparing a first lost model having an outer contour substantially corresponding to a desired interior contour of the finished product, depositing a metallic or ceramic material or a mixture thereof over the surface of the first lost model by spraying to form a layer defining a hollow core block, placing the hollow core block in a first mold, pouring a material for forming the second lost model into the first mold, coating a refractory material over the second lost model to form a second mold for casting, removing the second lost mold to form a cavity, casting a molten metal or alloy into the cavity and staving the mold to take out a finished product.
It would be advantageous, therefore, to provide a wax-based material that could be used at temperatures of up to 350.degree. F. without melting or softening sufficiently to deform.
The primary object of the invention is to provide a wax-based material that could be used at temperatures of up to 350.degree. F. to form ultra-lightweight radomes.
Another object of the present invention is to provide a material that could be used at temperatures of up to 350.degree. F. that does not require the use of metallic particles, such as lead particles, which have been found to have a deleterious effect on the electronic transmission and reception of the radome.
Additional objects and advantages of the invention will be more fully understood and appreciated with reference to the following description.